?I have no idea what to do for this problem despite the answer key \"explaining\

Question

?I have no idea what to do for this problem despite the answer key "explaining" it aomewhat. Please show and tell me step by step what to do as if explaining to somebody who never took chemistry before. I have no idea how why nor when to change the PV=NRT equation to what it was changed to. Please explain.

5. Feeling Gassy (10 pts) a) Buddy the Elf discovered an unknown gas (15.6 mg) in Professor Snape's potions room in a 100 mL balloon at 25 0C with a Pmeasured of 725 torr. What is the molecular weight of the gas? PV nRT mw 72 Store L. ath 0 torc K. ml Mw O. 54 0.38 2. MAW M w 4.00.9 mol Page Total

Explanation / Answer

given
P = 725 torr = (725/760) atm = 0.9539 atm
V = 100 mL = 0.100 L
T = 25 oC = (25 + 273) K = 298 K
R = 0.0821 atm.L/mol.K

1st find the number of moles of gas using
P*V = n*R*T
0.9539 * 0.100 = n*0.0821*298
n = 3.899*10^-3 mol

this is the number of moles of gas filled

mass = 15.6 mg = 0.0156 g

to find the molecular weight use formula,

n = mass/molecular weight
molecular weight = mass/n
= 0.0156 g / (3.899*10^-3 mol)
= 4.00 g/mol

Answer: 4.00 g/mol

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